The maximum recommended charge rate for most lipo batteries is 1C, which translates to the capacity of the battery expressed in mA (milliAmps).

Thus for your 640 mAh pack, the maximum recommended charge rate is 640 mA. If your charger does not allow you to select the exact rate, always select a lower rate. So with your charger, you'll want to select the 500 mA rate.

At a 1C charge rate, any size lipo will take a little over an hour to charge, assuming the pack is more or less fully depleted. You can always charge a lipo at less than its maximum 1C rate. It will simply take longer to charge, but it will still reach a full charge. Charging your 640 mAh pack at 500 mA will take about 90 minutes.

- Jeff